MUTATIONS

Fitness of MutationsThe fitness of a mutation describes its value to the survival and reproductive success of the organism.A mutation may turn out to be:

Lethal: Many mutations are lethal and embryos are non-viable.

Harmful: Non-lethal mutations, e.g. Down syndrome and sickle cell disease, may be expressed as effects that lower fitness.

Silent (neutral): Most point mutations are probably harmless, with no noticeable effect on the phenotype.

Beneficial (useful): Occasionally mutations may be useful, particularly in a new environment, e.g. insecticide resistance in insects, antibiotic resistance in bacteria.

A. HEREDITY – EUKARYOTIC CHROMOSOMES

B. MOLECULAR GENETICS – DNA/RNA

Gametic Mutations Somatic Mutations

Location of Mutations

The location of a mutation determines whether or not it will be inherited.

Most mutations occur in somatic cells and are not inherited.

Gametic mutations occur in the cells of the gonads (which produce sperm and eggs) and may be inherited.

Sperm

Egg

Fertilisation

Cleavage. Prior to implantation

Fetus

Baby

Cells of tissues

affected by the

mutation

EggSperm

Somatic mutations occur in body

cells. They are not inherited but may

affect the person during their

lifetime.

Gametic mutations are inherited

and occur in the testes of males

and the ovaries of females.

Mutation

Mutation

Mutation

Mutation

Neutral Mutations

Neutral mutations are hard to detect because they produce little or no change in the phenotype.

They may have little or no effect on the survival of an organism or its ability to reproduce.

They may be the result of a ‘same-sense’ mutation where a change in the third base of a codon still codes for the same amino acid.

Normal DNA

mRNA

Amino acids

Amino acid sequence from the non-

mutated DNA forms a normal polypeptide

chain

Mutation: Substitute C instead of T

Mutant DNA

mRNA

Amino acids

Despite the change in the last base of a

triplet, the amino acid sequence is unchanged

Beneficial Mutation Example

Tolerance to high cholesterol levelsin humans

In the small village of Limone, about 40 villagers have extraordinarily high levels of blood cholesterol, with no apparent harmful effects on their coronary arteries.

The village has a populationof 980 inhabitants and was,until recently, largely isolatedfrom the rest of the world, withsheer cliffs behind the village,the lake in front of them,and no road access.

Limone

Lake

Garda

Verona

Brescia

Italy

The village of Limone, on

the shore of Lake Garda,

Italy

High blood cholesterol and dietary fat

are implicated in the formation of

plaques in the coronary arteries and in

the development of cardiovascular

disease.

Beneficial Mutation ExampleThe 40 villagers possess a point mutation which alters the protein produced by just one amino acid. This protein is ten times more effective at mopping up excess cholesterol.

No matter how much excess cholesterol is ingested, it can always be disposed of.

All carriers of the mutation are related and have descended from one couple who arrived in Limone in 1636.

Generally, the people of Limonelive longer and show a highresistance to heart disease.

Point Mutations 2As a reference for the following screens, the diagram below illustrates the transcription and translation of DNA without a point mutation.

Original Unaltered Code

Transcription

Amino acid sequence forms a normal polypeptide chain

Translation

Original DNA

mRNA

Amino acids

Missense SubstitutionA single base is substituted by another.

Usually results in coding for a new amino acid in the polypeptide chain.

If the third base in a triplet had been substituted, the resulting amino acid may not be altered (due to degeneracy in the code).

Mutation: Substitute T instead of C

Polypeptide chain with wrong amino acid

Original DNA

Mutant DNA

mRNA

Amino acids

Nonsense SubstitutionA single base is substituted by another.

This results in a new triplet that does not code for an amino acid.

The resulting triplet may be an instruction to terminate the synthesis of the polypeptide chain.

Mutation: Substitute A instead of C

Original DNA

Mutant DNA

mRNA

Amino acidsMutated DNA creates a STOP codon which prematurely ends synthesis of the polypeptide chain

Sickle Cellscontaining mutant

hemoglobin (less soluble)

Normal Red Blood Cells

containing normal hemoglobin (soluble)

Sickle Cell MutationThe mutation responsible for causing sickle cell disease is a point substitution mutation.

Hemoglobin moleculesare made up of 2 α-chains

and 2 β-chains linked together

β-Chainhemoglobin

The sickle cell mutation involves the substitution of one base for another in the HBB gene, causing a single amino acid to be altered.

Hemoglobin clusters together to form fiber, which deform the red blood cells into a sickle shape

Beta (β) chain Alpha (α) chain

Normal base: TSubstituted base: A

DNA

Codes for the 1st amino acid

First base

Sickle Cell DiseaseSynonym: Sickle cell anemiaIncidence: Most common in people of African ancestry.

West Africans: 1%(10-45% are carriers)

West Indians: 0.5%

Gene type: Autosomal recessive mutation (HBB) on chromosome 11 which results in the substitution of a single nucleotide in the HBB gene coding for the beta chain of hemoglobin.

Gene location: Chromosome 11HBB

p q

Normal red blood cells

Sickle-cell

Photo Defiers.com

Sickle Cell Disease

Symptoms include the following:

Pain, ranging from mild to severe, in the chest, joints, back, or abdomen

Swollen hands and feet

Jaundice

Repeated infections, particularly pneumonia and meningitis

Kidney failure

Gallstones (at an early age)

Strokes (at an early age)

Anemia.

Reading Frame Shift by InsertionA single base is inserted, upsetting the reading sequence for all those after it.

A reading frame shift results in new amino acids in the polypeptide chain from the point of insertion onwards.

The resulting protein will be grossly different from the one originally encoded (it is most likely to be non-functional).

Large scale frame shift results in a new amino acid sequence. The resulting protein is unlikely to have any biological activity.

Mutation: Insertion of C

Original DNA

Mutant DNA

mRNA

Amino acids

Partial Reading Frame ShiftA single base is inserted and another is deleted at a different location. This causes a localised frame shift.

The amino acid sequence between these points changes.

Depending on how many amino acids are affected, the resulting protein may have some biological activity.

Altered chain which may or may not produce a protein with biological activity

Mutation: Insertion of C Mutation: Deletion of C

Original DNA

Mutant DNA

mRNA

Amino acids

Cystic Fibrosis MutationThe mutation causing 70% of cystic fibrosis cases is a gene mutation (delta F508) involving a triplet deletion.

Base 1630

Part of the DNA sequence in the

CFTR gene

This triplet codes for the 500th amino acid

Cl-

Cl-Cl-

Outsidethe cell

Cellcytoplasm Cl-

Normal CFTR proteinregulates chloride transport

across the membrane

The 508th triplet is absent in the mutant form

Cellmembrane

Mutant CFTR proteincannot regulate chloride transport. Chloride

ions to remain in the cell and water enters the cell

Cellcytoplasm

Outsidethe cell

Water

CFTR protein

Cl-Cl- Cl-

Cl-Cl-Cl-

Cl-

Cystic FibrosisSynonyms: Mucoviscidosis, CF Incidence: Varies with populations:

Asians: 1 in 10 000

Caucasians: 1 in 20-28 are carriers

Mutation type: Autosomal recessive. Over 500 different recessive mutations of the CFTR gene have been identified:

deletions, missense, nonsense, terminator codon

Gene locationChromosome 7

CFTR

q

p

A classic example

Sickle-cell anemia: hemoglobin B gene (HBB)Aa heterozygoes partially protected against malariaBut aa homozygotes suffer adverse health effectsHeterozygote advantage -- but only where malaria is present

See: anthro.palomar.edu/synthetic/synth_4.htm

humans and chimpanzeesdiffer in disease outcomesand susceptibility in all 7

What are the biggest public health concerns?

4 / 7 are associated with differences in diet betweenthe two species

Thus, the chimpanzee genome holds important clues to understanding the genetic basis for these diseases in humans.

Heart disease

Diabetes

Infectious disease

Cancer

Obesity

Stroke

Neurodegenerative disease

Block Mutations: Deletion

Break occurs at two points on the chromosome and the middle piece falls out.The two ends rejoin to form a chromosome deficient in some genes.Alternatively, the end of a chromosome may break off and be lost.

Chromosomerejoins

Break

Break

Genes

Step 1 Step 2 Step 3

Segmentis lost

Deletion ExampleHuman chromosome 1 shows two forms of deletion.

These may involve deletion of either a chromosome tip (left) or a middle segment with the tip rejoined (right).

Deletions involving small amounts of chromosomal material underlie several disorders, including:

Cri-du-chat syndrome

Prader-Willi syndrome

Angelman’s syndrome

Deletions of large amounts of chromosomal material areusually lethal.

Tip deletion Mid-segment deletion

Before After Before After

Tiprejoins

LostLost

1

1

1

1

Block Mutations: Translocation

Translocation involves the movement of a group of genes between different chromosomes.A piece of one chromosome breaks offand joins on to another chromosome.The result is a chromosome deficient in genes and one with too many genes.

Segmentremoved

Segments

join

Break

Genes

Step 1 Step 2 Step 3

Translocation Example

Translocation can occur between human chromosomes 9 and 22.The tips of the two chromosomes are exchanged.This is the translocation observed in chronic myeloid leukemia.

Before translocation After translocation

22

9

The tips of the chromosomes swap

22

9

Block Mutations: InversionThe middle piece of the chromosome falls out, rotates through 180°, and then rejoins.There is no loss of genetic material.

Break

Break

Genes

Segmentrotates 180°

Step 1 Step 2 Step 3

Segmentrejoins

Inversion ExampleA segment of human chromosome 2 is inverted (caused by looping of the chromosome)All inversions cause abnormalities during meiosis and affect the viability of the gametes produced.However, if a combination of genes within an inversion is desirable, they can act as a supergene and can confer a selective advantage.

Normal Inversion

Flip

22

Block Mutations: Duplication

A segment is lost from one chromosome and is added to its homologue.

The chromosome on the left was the 'donor' of the duplicated piece of chromosome.

The chromosome with the duplication will become incorporated into a gamete, which may later contribute to an embryo.

Joins on tohomologouschromosome

Segmentremoved

Break

Genes

Step 1 Step 2 Step 3

Duplication ExampleA segment of human chromosome 9 is duplicated.

A segment is taken from its homologue and inserted to produce double copies of some genes.

Some genes may be disrupted.

Gene duplications can have evolutionary significance.

Example: The alpha and beta chains of hemoglobin arose following a duplication event 500 million years ago.

Normal Duplication

A segment is tansferred from

one chromosome

into its homologue

Duplicatesegment

Identicalsegment

9

9

Maternal Age EffectMany aneuploidies show a ‘maternal age effect’, where incidence increases with the age of the mother.

Example:Down syndrome is 100 times more likely in children of mothers over 45 years, than in those of mothers less than 19 years.

Est

imat

ed r

ate

of

Do

wn

Syn

dro

me

(per

100

0 bi

rths

)

Maternal age in years

Maternal age(years)

< 3030 - 3435 - 3940 - 44

> 44

Incidence per1000 live births

< 11 - 22 - 55 - 1010 - 20

1 in 2 300 1 in 880 1 in 290

1 in 100

1 in 46

Causes of Maternal Age EffectMaternal age effect probably arises because:

All eggs are present at birth but are suspended in their development in early prophase until puberty.

A woman, on average, will produce about 400 eggs in her lifetime (12 per year).

Therefore, by the end of her reproductive life, the egg cells that remain are old and there is a greater chance that errors in meiosis will occur.

A similar, though less marked effect is exerted by the age of the father.

Sperm from older men have a slight tendency to be deficient in chromosomes

Older egg cells are more prone to faulty meiosis

Sex chromosome aneuploidies

Male ..............1 427

Female .............422

Autosomaltrisomics

Trisomy 13 .......... 42

Trisomy 18 .........100

Trisomy 21 ......1041

Otherabnormalities

Total .......2133

Trisomics .......39 000 XO .................13 500Triploids .........12 750Tetraploids .......4500Others ..............5250

The Fate of ConceptionsFor every million conceptions that occur, a significant number have genetic abnormalities and fail to develop into a completely normal child:

Conceptions1 000 000

With chromosome abnormalities

5165

Perinatal deaths17 000

Children833 000

Other causes75 000

Chromosome abnormalities

75 000

Spontaneous miscarriages150 000

Live births850 000